This invention relates to an aft seal for industrial gas turbine combustion systems, and more particularly, to an integral aft seal using xe2x80x9cExe2x80x9d seal technology to provide variable thickness to the seal to flexibly and resiliently seal the interface between the transition piece and the first stage nozzle.
In gas turbine applications it is essential to seal the interface between two relatively movable mechanical members which are in close proximity to one another. For example, it is advantageous from a performance point of view to seal the interface between a transition piece and the first stage nozzle as this interface is prone to the leakage of gases. An aft seal is generally composed of multiple separate parts which are arranged along the periphery of the transition piece. When the turbine is in operation high temperature gases flow through this interface and cause thermal growth. The aft seal thus must be flexible enough to seal even when the interface changes in shape and/or size due to thermal growth. A seal between these two members has to have two characteristics in order to be successful: flexibly seal between relatively moving pieces in a high temperature environment, and be resistant to wear. These characteristics are difficult to achieve in a single material.
Conventionally, the aft seals that are used in gas turbine applications are made of four separate pieces: inner and outer floating seals and two side seals. The inner and out floating seals engage slots machined into the transition piece aft frame and the first stage nozzle. The side seals engage matching slots in the aft frames of adjacent transition pieces. This seal design is referred to as a floating seal due to the fact that the individual pieces are allowed to move relatively freely in their respective slots. The floating seal design is a success and lasts up to 8,000 hours in operation. These conventional floating seals, however, are limited in their applicability in a new generation of machines which are being engineered for 24,000 hours of operation between inspections. There are two main shortcomings of the floating seals: firstly, the four separate pieces do not provide maximum sealing particularly at the corners where the individual pieces come together; and secondly, the seals and aft frame slots in which they are received experience excessive mechanical distress. Major repairs are generally needed after each combustion cycle, 6500 to 8000 hours. Also, since the bulky transition pieces are generally assembled into the turbine machine in cramped quarters, proper engagement of the inner and outer floating seals into the first stage nozzle slots is often difficult to achieve.
Any improvements to the aft seal which will enable longer combustion cycles can significantly reduce operation costs by reducing the number of maintenance inspections which result in downtime. In addition, improved sealing in the aft interface between the transition piece and the first stage nozzle will significantly improve turbine efficiency.
An integral aft seal of the present invention is a single piece seal that is comprised of a corrugated seal assembly and side seal supports. The corrugated material in the seal is referred to as the xe2x80x9cE-sealxe2x80x9d material for convenience. The corrugated seal assembly is comprised of the E-seal material and a stiffener attached on each planar face of the E-seal to act as wear surfaces. The overall planar shape of the E-seal assembly corresponds to the shape of the transition piece aft frame. The E-seal is a single piece of material that is corrugated into a spring-like shape arranged to provide variable thickness to the E-seal. The stiffeners attached on the faces of the E-seal bear against the turbine components in the interface to provide wear resistance. The E-aft seal is attached to a turbine component, preferably the transition piece, by an appropriate type of fastener so that there is no relative motion at this juncture and therefore no wear of the transition piece aft frame. In place between the transition piece and the first stage nozzle in this manner, the single piece E-seal assembly seals against leakage of gases even at the corners of the aft frame by flexing as necessary to maintain contact with the first stage nozzle even when the components move dynamically relative to one another.
The side seals of conventional aft seals are replaced in the present invention with supports that are attached to the nozzle retaining ring by loose fitting bolts. The supports are pressed firmly against the inner and outer radials of the first stage nozzle and held there by the compressive loading of the E-seals in the final assembly. In this manner, relative motion between the side supports and the nozzle are minimized which reduces wear, and the nozzle and retaining ring are allowed to grow thermally at different rates without causing thermo-mechanical distress to the mating parts. Each side seal support has two wings protruding from each side which fit snugly between the inside surfaces of the inner and outer radials. Two compliant seals per support seal this interface from leakage. When the supports are properly placed, these wings in combination with adjacent support and the first stage nozzle inner and outer radials form a continuous sealing surface for the E-seal. The E-seal is loaded against this surface to provide the necessary seal of compressor discharge air. The spring-like characteristic of the corrugated E-seal helps maintain constant contact between the seal, the nozzle inner and outer radials and the sealing surfaces of the supports at all times by flexing to adjust its thickness. The variable thickness of the E-seal ensures that the interface that it spans is consistently sealed.
To alleviate the effects of thermal loading on the supports due to stagnation of flow on the back side, cooling holes and slots are provided on the supports at the sealing surfaces. Alternatively, different materials such as high temperature superalloys or engineered ceramics could be used for the components to eliminate the need for cooling air which would make the air available for combustion instead. Also, by employing a superalloy or similar material, the thermal growth of the supports will be better matched with the first stage nozzle. The thermal growth of the supports and first stage nozzle can be matched closely enough to eliminate the need for compliant seals at that interface.
This E-seal for the aft frame between the transition piece and the first stage nozzle possesses a significant wear resistance advantage over conventional aft seals, and provides an improved seal of the interface for better efficiency. In addition, the single piece aft seal assembly simplifies installation during initial assembly and inspection and replacement during periodic maintenance stoppages to provide a more reliable seal and save operating costs. By rendering the aft frame almost impervious to wear, the E-seal can lengthen the cycle time by three times, up to 24,000 hours. This greatly reduces the frequency of inspections, and therefore turbine downtime.